1. Field of the Invention
The present invention relates to methods for preparing curable pigment inkjet ink sets and curable pigment inkjet ink sets having improved properties of colour gamut, dispersion quality and stability.
2. Description of the Related Art
In inkjet printing tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the ink drops image-wise onto the ink-receiver.
Inkjet ink compositions typically include following ingredients: dyes or pigments, water and/or organic solvents, humectants such as glycols, detergents, thickeners, polymeric binders, preservatives, etc. It will be readily understood that the optimal composition of such ink is dependent on the inkjet printing method used and on the nature of the ink-receiver to be printed. The ink compositions can be roughly divided in:                water-based, the drying mechanism involving absorption, penetration and evaporation;        oil-based, the drying involving absorption and penetration;        solvent-based, the drying primarily involving evaporation;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification;        UV-curable, in which drying is replaced by polymerization.        
Water-based inks suffer from a number of disadvantages such as:
(a) their requiring water evaporation and therefore an extensive drying system, especially when printing speed is important;
(b) the tendency of large printed areas to cockle;
(c) sensitivity of images to wet and dry rubbing; and
(d) the tendency of low viscosity inks to dry at the tip of the orifice.
The use of polar solvent-based inks can overcome some of the problems inherent in water-based inks, but results in other problems such as the possible generation of toxic or inflammable vapours. Therefore efforts were made to develop low-solvent ink compositions from which the concept of radiation curable ink compositions emerged.
Radiation curable pigment inkjet inks comprise one or more colorants, polymerizable compounds and one or more photo-initiators. The photo-initiator absorbs radiation thereby generating a radical (radical polymerization) or an acid (cationic polymerization) which initiates the polymerization reaction of the polymerizable compounds.
The colorants in curable pigment inkjet inks are usually colour pigments because they exhibit better light and ozon stability compared to dyes. Preparation of thermally stable dispersions of submicron colour pigment particles is more difficult for curable pigment inkjet inks, especially when the colour pigments have a non-polar surface.
Often curable inkjet ink sets are now commercialized, wherein the different inks are not sufficiently matched with each other. For example, the combined absorption of all inks does not give a complete absorption over the whole visible spectrum, e.g. “gaps” exist between the absorption spectra of the colorants. Another problem is that an ink might be absorbing in the range of another ink. The resulting colour gamut of these inkjet ink sets is low or mediocre.
Generally radiation curable pigment inkjet ink sets exhibit a mediocre colour gamut compared to aqueous pigment and dye based inkjet ink sets. The colour gamut represents the number of different colours that can be produced by an ink set. Several methods for improving colour gamut are known in aqueous inkjet jets but have disadvantages when used in radiation curable pigment inkjet inks. For example the combination of pigments with dyes or milling pigments to smaller particle size improve the colour gamut at the expense of light stability. In aqueous inks this is at least in part compensated by adding UV-absorbing compounds to the ink. But, including UV-absorbers in radiation curable pigment inkjet inks result in lower curing speeds since the photo-initiators usually absorb UV-light.
The colour gamut of ink sets can also be improved by selecting, matching and mixing different colour pigments. However, this is a time-consuming method, not only by the numerous combinations that can be made, but also by the above mentioned problems for the preparation of thermally stable radiation curable dispersions of submicron colour pigment particles. Furthermore, a change of pigment may require adaptation of the photo-initiator (system) due to large changes in the absorption of the pigment in the UV-region of the spectrum.